Mag Marius Vorstman

Description of dehydration performance of amorphous silica pervaporation membranes

The dehydration performance of a ceramic pervaporation membrane is studied for the separation of isopropanol/water mixtures. The membranes are provided by ECN (The Netherlands) and consist of a water selective amorphous silica top layer and four alumina supporting layers. For the system investigated, these membranes appear to combine high selectivities with high permeabilities. This results in a very high pervaporation separation index (PSI up to 6000 kg/m2 h at 80°C). A generalized Maxwell–Stefan model has been set up to model the fluxes. From this analysis it follows, that the water flux is only proportional to its own driving force. It is experimentally demonstrated that this holds for a wide range of operating conditions and feed compositions. From these data, values of various Maxwell–Stefan diffusivities are estimated.

Properties of high flux ceramic pervaporation membranes for dehydration of alcohol/water mixtures

In this paper, a set of performance data of a ceramic pervaporation membrane, provided by ECN, Petten, The Netherlands, is described. For the dehydration of alcohol/water mixtures, these membranes appear to combine high selectivities with high permeabilities, resulting in a high Pervaporation Separation Index (PSI). At 70°C the water flux and separation factor for the dehydration of isopropanol (water concentration varied between 1 and 7 wt.%) range from 0.45 to 2.8 kg/(m2 h), and 340–600, respectively. For the dehydration of n-butanol (water concentration varied between 1 and 5 wt.%) these values are between 0.4 and 2.3 kg/(m2 h) and 680–1340, respectively. These flux values are high as compared with the ceramic pervaporation membranes described in the literature.

Opportunities for process intensification using reverse micelles in liquid and supercritical carbon dioxide

Liquid and supercritical carbon dioxide can be used as a replacer for organic solvents, which potentially enables the development of clean processes. A general disadvantage of CO2, however, is that it is a very poor solvent for high molecular weight or hydrophilic molecules. By using reverse micelles to overcome this problem, a wide variety of novel processes can be thought of. In this review, it is shown that the use of micellar systems in supercritical CO2 leads to processes generating a minimum amount of waste and with a low energy requirement. It is also shown that especially downstream processing of biochemicals and polymerization reactions can benefit from this technique, as the number of consecutive process steps can be reduced significantly.

Unsteady-state flux behaviour in relation to the presence of a gel layer*

Abstract The unsteady-state flux behaviour has been studied for silica and dextran in a stirred ultrafiltration cell. Under the experimental conditions dextran and silica show a clearly different flux behaviour. During the filtration of dextran only a polarization layer is build up. For silica also a gel layer formation occurs. As a result the time to reach steady-state flux is less than a minute for dextran, whereas the formation of the silica gel layer takes more than one hour. The osmotic pressure model provides a good description of the flux for the experiments with dextran. If mass transfer coefficients are used which are higher than those electrochemically measured the transient flux for silica can be rather well predicted by the gel-polarization model. The use of flux measurements the presence of a gel layer is discussed.

Solute rejection in the presence of a deposited layer during ultrafiltration

During ultrafiltration deposited layers are often formed on the membrane surface. These layers not only reduce the volumetric flux through the membrane, but also may influence the rejection of other solutes in the feed. In the present paper we will show that besides an increase in the rejection, a decrease in rejection may also occur, which can completely alter the aimed selectivity of the separation process. The influence of deposited layers has been studied experimentally by two types of depositing components: silica sol and the protein BSA. In the presence of a relatively open silica deposit a strong drop in the rejection of PEG and dextran was found compared to the rejection on a clean membrane. For thick deposit layers the rejection even decreased to zero, thus resulting in a total permeation of a normally partially rejected solute. On the other hand an increase in PEG rejection occurred in the presence of a BSA deposit. Due to the compressibility of the protein deposit the highest rejections were measured at the highest pressures. The effects were the most pronounced at the isoelectric point of BSA. A model is presented to describe the underlying phenomena.

Design directions for composite catalytic hollow fibre membranes for condensation reactions

Pervaporation membrane reactors are ideal candidates to enhance conversion in reversible reactions generating water as a by-product. The equilibrium displacement can be enhanced by catalytic membranes due to the close integration of reaction and separation. In this paper, the viability of composite catalytic hollow fibre pervaporation membranes for condensation reactions is examined. The esterification reaction between acetic acid and butanol has been taken as a model reaction, for which a parametric model study was carried out to provide a fundamental understanding of the composite catalytic membrane reactor behaviour. With increasing catalytic layer thickness, the conversion becomes no longer limited by the amount of catalyst present in the reactor but by diffusion in the catalytic layer. External mass transfer was never found to be rate-limiting. An optimum catalytic layer thickness was found to be around 100 μm under the prevailing conditions, which is within practically reachable dimensions. At this optimum catalytic layer thickness, the performance of a catalytic membrane reactor exceeds the performance of an inert membrane reactor due to the close integration of reaction and separation. This shows the potential added value of such a membrane system compared with more usual reactor designs. The exact value of this optimum is a function of the reaction kinetics and the membrane permeability.

High-flux palladium-silver alloy membranes fabricated by microsystem technology

In this study, hydrogen selective membranes have been fabricated using microsystem technology. A 750 nm dense layer of Pd (77 wt%) and Ag (23 wt%) is deposited on a non-porous 1 mm thick silicon nitride layer by cosputtering of a Pd and a Ag target. After sputtering, openings of 5 μm are made in the silicon nitride layer to create a clear passage to the Pd/Ag surface. As a result of the production method, these membranes are pinhole free and have a low resistance to mass transfer in the gas phase, as virtually no support layer is present. The membranes have been tested in a gas permeation system to determine the hydrogen permeability as a function of temperature, gas flow rate, and feed composition. In addition, the hydrogen selectivity over helium has been determined, which appears to be above 1500. At 0.2 bar partial hydrogen pressure in the feed, the hydrogen permeability of the membranes has been found to range from 0.02 to 0.95 mol.H2/m2×s at 350 and 450°C, respectively. It is expected that by improving the hydrodynamics and increasing the operation temperature, substantially higher fluxes will be attainable.

Experimental investigation of CaSO4 crystallization on a flat plate

The scaling of calcium sulfate was studied by performing laboratory experiments under controlled conditions. The experiments were aimed at measuring the rate of deposition at different positions on a heated surface. The overall thermal resistance was determined from temperatures measured using thermocouples positioned in the bulk fluid and the wall of the heated plate. Calcium sulfate was used as the experimental fluid. It was observed that nucleates started forming on the downstream side. A nucleation front was formed, and it was seen to move from the downstream to the upstream side. The rate of growth as a function of position was observed to increase with the initial wall temperature distribution, resulting in a final thickness of the scale layer that increases accordingly. While the rate of growth was found to be independent of flow velocity for Reynolds numbers of 11,000 and 23,000, the results showed that the rate of growth decreased by about 20% for Re == 34,000. Further, the induction period is reduced by increasing the flow velocity. An increase in the degree of supersaturation also reduces the induction period. It is concluded that scaling due to CaSO4 results in a non-uniform porous scale layer with a profile that mimics the initial surface temperature.

A novel process for the catalytic polymerization of olefins in supercritical carbon dioxide

A novel process is being developed for the catalytic polymerization of olefins in supercritical carbon dioxide. Potential applications will mainly be in the production of EPDM and other elastomers. For this purpose, catalysts have been synthesized and tested. Late transition metal-based catalysts of the Brookhart type have been used to polymerize 1-hexene and ethylene in supercritical CO2 yielding high molecular weight polymers. Additionally, polymerizations of 1-hexene in CH2Cl2 have been performed as reference experiments. In the case of 1-hexene, a comparison with the polymerization behavior in CH2Cl2 reveals similar molecular weights and molecular weight distributions. Furthermore, the multicomponent phase behavior of polymer systems at supercritical conditions has been studied. The phase behavior of binary and ternary systems containing poly(ethylene-co-propylene), ethylene and CO2 has been determined experimentally by measuring cloud-point isopleths. The predictions of the phase behavior as obtained from statistical associated fluid theory calculations agree very well with the experimentally determined cloud-points. Based on these results, some important aspects for the process design have been addressed, for which catalyst solubility, efficient recycle of CO2 and purification of the polymer product are key issues.

Polymerisation of olefins catalysed by a palladium complex in supercritical carbon dioxide

A late transition metal catalyst has been used to polymerise hex-1-ene and ethene in supercritical carbon dioxide, yielding high molecular weight polymers; a comparison with polymerisations in CH2Cl2 reveal that polymers with identical molecular weight and polydispersity are formed.